Text by Vivian Kuusk

Lots of species grow different looking leaves when they are young and adult. For example young conifers look more like little soft brushes rather than the forest giants, and lots of common houseplants look totally different in their natural habitat, because on our windowsills they produce only juvenile leaves (good example is devil`s ivy – Epipremnum aureum). The phenomena of juvenility has been a question for quite some time and we wanted to understand which morphological, anatomical and chemical changes occur in the needles upon juvenile-to-adult transition. As Mediterranean pines tend to keep juvenile needles longer than pines in temperate zone, stone pine (Pinus pinea), Aleppo pine (Pinus halepensis) and black pine (Pinus nigra) were chosen for the investigation. We found that juvenile needles are narrower, contain less dry mass per area and have thinner cell walls. Nitrogen content per dry mass was similar in all plant ages, but the photosynthetic tissue and size of chloroplasts was bigger in juvenile leaves. So we suggest that the ecological advantage of having juvenile leaves is of maximum carbon gain and establishing the saplings. The role of mature type leaves is to be more durable against various stress conditions, like for example mechanical stress from wind. I´d like to draw a parallel with human homes here – if you are young and don´t have lots of money, you live in a cheap condo, but when you have a bit more, you start thinking of building a house that would last a lifetime or more. Therefore plants invest more into growing durable leaves when they are older and have established their above- and underground parts well enough.

Pine (Pinus) species exhibit extensive variation in needle shape and size between juvenile (primary) and adult (secondary) needles (heteroblasty), but few studies have quantified the changes in needle morphological, anatomical and chemical traits upon juvenile-to-adult transition. Mediterranean pines keep juvenile needles longer than most other pines, implying that juvenile needles play a particularly significant role in seedling and sapling establishment in this environment. We studied needle anatomical, morphological and chemical characteristics in juvenile and different-aged adult needles in Mediterranean pines Pinus halepensis Mill., Pinus pinea L. and Pinus nigra J. F. Arnold subsp. salzmannii (Dunal) Franco hypothesizing that needle anatomical modifications upon juvenile-to-adult transition lead to a trade-off between investments in support and photosynthetic tissues, and that analogous changes occur with needle aging albeit to a lower degree. Compared with adult needles, juvenile needles of all species were narrower with 1.6- to 2.4-fold lower leaf dry mass per unit area, and had ~1.4-fold thinner cell walls, but needle nitrogen content per dry mass was similar among plant ages. Juvenile needles also had ~1.5-fold greater mesophyll volume fraction, ~3-fold greater chloroplast volume fraction and ~1.7-fold greater chloroplast exposed to mesophyll exposed surface area ratio, suggesting overall greater photosynthetic activity. Changes in needle traits were similar in aging adult needles, but the magnitude was generally less than the changes upon juvenile to adult transition. In adult needles, the fraction in support tissues scaled positively with known ranking of species tolerance of drought (P. halepensis > P. pinea > P. nigra). Across all species, and needle and plant ages, a negative correlation between volume fractions of mesophyll and structural tissues was observed, manifesting a trade-off between biomass investments in different needle functions. These results demonstrate that within the broad trade-off, juvenile and adult needle morphophysiotypes are separated by varying investments in support and photosynthetic functions. We suggest that the ecological advantage of the juvenile morphophysiotype is maximization of carbon gain of establishing saplings, while adult needle physiognomy enhances environmental stress tolerance of established plants.